It was both awful when it showed up in the enterprise and amazing at unleashing creativity for many. Most young non-technical people I knew during its rise had regularly made Flash creations or even games, and deeply enjoyed the Cambrian explosion of games and animations for a few years.
It was really meant for animation and games but got misused as a web GUI tool. I think it would've been fine to allow it anyway, and anyone who wants to build a GUI can just not use Flash.
Flash players had zoom built in. And I believe there were textareas that allowed people to copy and paste text if they wanted, though it wasn't very common
Flash was the last thing that got people excited for the Web generally
Flash was the original web Excel (also Lotus 1-2-3) -- a simultaneous design + data + programming tool.
These are terrible for maintainability, but excellent for usability.
On the whole, I'd say it was easily a loss for the greater web that web programming left the citizen-programmer behind. (By requiring them all to turn into hamfisted front-end javascript programmers...)
Many of the centralized evils of the current web might have been avoided if there had remained an onramp for the neophyte to really create for the web.
I.e. Facebook et al. might have instead been replaced by a hosted, better-indexed Macromedia create + edit + host platform
Or the amount of shit code produced by inexperienced front-end devs throwing spaghetti at IE might have been reduced
It was a plague on the web, you couldn't zoom, select text, go back, just a black box ignoring everything about your web browser.
Killing it was probably the best thing Jobs ever did.